Zinc smelting



June 3, 1952 R. K. wARlNG ET Ax. 2,598,743

` ZINC SMELTING Filed Jan. 12, 1950 CDC) FIG,

ZoTHr-:R SLAG coNsTlTurfzNTs [loZ-(ZLIMm/Z'SILICM] oliva valus- :awl'lINVENTOR S BY Zak?, M, v TTMM/EYS Patented June 3, 1952 ZINC SMELTINGRobert K. Waring, Luther D. Fetterolf, and Thomas L. Hurst, Palmerton,Pa., assgnors to The New Jersey Zinc Company, New York, N. Y., acorporation of NewJersey Application January 12, 1950, `Serial No.138,136

(Cl. 'i5-14) Claims. l

This invention relates to thesmelting of oxidic zinciferous materialand, more particularly, to the electric `arc furnace smelting of oxidicvzinciferous ore.

Because of its inherent simplicity and the absence of such requirementsas vspecial charge preparation, the electric arc furnace smelting ofzinc has attracted much .thought and speculation. Numerous proposalshave been advanced heretofore for the electric furnace smelting of zinc,but to the best oi .our knowledge those proposals which have beenreduced to practice have not been successful in producing massive zincmetal. These consistently discouraging results over a period of decadeshave not been caused by the diculty of smelting the zinc ore becausesuch an ore can be smelted with facility at the temperatures which arereadily obtained in an electric furnace. The diiculty encountered'heretofore has resided in the condensationof the zinc vapor produced byelectric furnace smelting. In 4spite of more optimistic predicitions,physical .embodiments of prior art proposals vhaveresnlted in theproduction of vast quantities of kblue powder to the substantialexclusion of massive metallic zinc.

In a general sense, the smeltinginethod of our invention, applied to thelsmelting ,of oxidic vzinciferous ore, resembles those of .the 'priorkart in that it produces three productszinc vapor, a molten slag, and ametallic iron product resulting from reduction of the -iron oxide whichinvariably accompanies the Zinc in its ores. As pointed out hereinafter,our method is also applicable to the smelting of substantially iron-freevoxidic zinciferous materials and in such oase the products of thesmelting operation do not include metallic iron. We have found, however,that when the smelting of oxidic'zinciferous material is carried out ata temperature Within the range of1250" to 1450 C. at the interfacebetween armoltenslag bath and a floating layer of the `zinciferouscharge, the zinc vapor produced by such smelting is substantially freefrom the dust-forming iml vcan successfully smelt zinciferous materialVin an electric arc furnace while substantially preventing thedevelopment of a temperature in excess of 1450" C. in any portion of theslag exposed to the furnace atmosphere, this 'temperaturexbeing measuredas the .temperature of the slag being tapped lfrom the `furnace. Thenecessary slag uidity can be insured by themaintenance ofthe slagcomposition within certain well-'defined limits in accordance withourinvention,

Oxidic zinciferous ores generally comprise zinc, cadmium, lead, copper,silver and iron, essentially in the form of oxides, each of which isreadily Areducible by carbonaceous ,material at temperatures Within therange of about l250 C. yto 1450 CV., as well as oxides of calcium,magnesium and silicon which are not readily reducible undertheseconditions. ln heating the entire mass of a smelting charge to atemperature Awithin such a range in an electric furnace, it is a normalcharacteristic of such an operation that a substantial portion of thecharge is vheated to an appreciably higher temperature. When a portionof a charge derived from the reduction of oxidic zinciferous ore isheated `to a temperature substantially kabove 1450 C., there is apronounced tendency for one or more of the gangue constituents-lime,magnesia and silica-as well as manganese oxide, if present, to bevolatilized either directly or indirectly, or both. These ,constituentsmay be volatilized directly in the form Vof the oxides per se, or theymay be volatilzed indirectly first by `reduction of the oxides to themetallic form, followed bv volatilization of the metals which are thenreoxidized by `Carbon monoxide and carbon dioxide in the furnaceatmosphere. Volatilization of these charge components in a relatively`hot portion of the charge is followed by their solidication in a coolerportion of the chamber, and the soiidied materials thereupon appearA inthe furnace atmosphere in the form of dust- `like particles. Theseparticles comprise the aforementioned dust-.forming impurities whichcontribute to the formation of blue powder and which indicate theexistence .of furnace conditions Which lead to the production of Ylargeamounts of blue powder instead of massive metallic zinc. `By maintenanceof a `molten slag temperature within the range of 1250" .to 1450" C.,which is made possible by control of the slag uidity in accordance withour invention, `local overheatingof the slag or charge to any `tern--perature substantially in excess of 15450 C. is

avoided and the production of .dust-forming impurities in the zincvapor-bearing smelting gases is inhibited.

Accordingly, it will `be observed `.that we have developed a method Aofsmelting an oxidic zinciferousiV material Vin the presence ofcarbonaceous reducing materialin' an electric arc furnace with Vtheresulting production of a molten slag and smelting gases bearingmetallic zinc vapor substantially free from dust-forming impurities. Inthe smelting of oxidic zinciferous ore, a molten iron product will alsobe obtained. The method of our invention comprises charging thezinciferous material and reducing material into the furnace in a looseand dry condition, establishing in the charge composition such relativeamounts -of the zinciferous and reducing materials as to effectreduction of at least a major portion of the zinc component of thecharge and such reduction of any iron present in the zinciferousmaterial (as in the case of a zinc ore) as topreclude the presence inthe molten slag of more than 6% by weight of iron oxide (calculatedasFe) further establishing in the charge composition such relativeproportions of calcareous and siliceous material, with respect to theother slagforming constituents, that upon smelting of the zinciferousmaterial the resulting molten slag will contain not more than 85 byweight of lime (CaOI) andy silica (SiO2) in a ratio between 0.9:1 and1.2:1 and not-exceeding theV ratio, with respect to the total amount ofthe other constituents of the slag, represented by the line AB in Fig. 1of the accompanying drawing, maintaining the slagin the molten conditionat a temperature of at least 1250 C. by electric arc heating, and`'smelting the dry charge on the surface of the molten slag at atemperature not in excess of 1450 C. The line AB in Fig. 1 represents anexperimentally determined critical relationship between Vthe lime-silicaratio and the total amount of other constituents present in the slag.Adherence to this relationship in the mannerl recited "assurestherequisite fluidity of the slag for effective practice of our smeltingmethod. However, optimum slag composition, corresponding t'o optimumslag fluidity, is obtained in accordance with 'our invention by soproportioning the aforementioned calcareous and siliceous material thatthe lime plus silica does not exceed 83% and the lime-silicaratio fallswithin the range of 1.05':1 and 1.15:1 and not exceeding, with respectto the total amount of other constituents of the slag, the ratiorepresented by the line CD in Fig. 1. In the interestl of a completeunderstanding of the invention, but not as any limitation thereof, itwill be described herein particularly with respect to the smelting of anoxidic zinciferous ore. In the course of our smelting operation, the

zinc, cadmium, lead, copper and silver oxides present in the ore arereadily reduced. Iron oxide in the ore is also largely reduced tometallic iron but it appears that some of the nascent iron tends toreduce the zinc oxide and thus become reoxidized to ferrous oxide.Although stable equilibrium conditions are not established in such acontinuous smelting operation, it is nevertheless a fact that somemetallic iron is formed simultaneously with the metallic Zinc and that,depending largely upon the amount of reducing material present, someferrous oxide remains unreduced in the slag. When the metallic iron isproduced in the medium of a suitably fluid slag and in the presence ofcarbonaceous reducing material, globules of carburized molten iron formin the slag and ultimately coalesce and tend to liquate therefrom in abody below the slag. The resulting body of molten iron normally containssuch an amount of dissolved carbon as to render it molten attemperatures as low as about 1150o to 1200 C. Inasmuch as the smeltingheat is supplied to the upper portion of the charge in an electric arcfurnace, the temperature prevailing in the lower portion of the furnacebelow the slag will generally be somewhat lower than that of the slagitself. In order to make possible continuous operation of the furnace,the metallic iron must be maintained in a tappable molten state whilethe smelting operation proceeds at a temperature within the range of1250 to 1450" C. The iron will have a melting point below 1450" C. if itcontains at least 1% to 2% carbon and will remain molten at atemperature between about 1150" and 1200 C. if it contains about 4%carbon. If the supernatant slag is not unduly o xidic, the metallic ironproduct will remain carburized to the extent necessary to maintain it inthe molten condition. Inasmuch as the presence of iron oxide in the slagimparts to the slag a certain oxidic character with respect to thecarburized metallic iron product, we have found it to be necessary tolimit the iron oxide content of the slag to a maximum of about 6% byWeight (calculated as Fe, which corresponds to 7.7% FeO) in order tomaintain the metallic iron product in a suitably Vtappable moltencondition, whereby the iron product and the slag will be separablytappable, while maintaining thereabove a smelting operation within thetemperature range of l250 rto 1450" C. The iron oxide content of theslag can be readily adjusted, as is well known in the art, by propercorrelation of the amounts of ore and carbonaceous reducingmaterialpresent in the furnace charge.

The smelting method of our invention is applicable to any oxidiczinciferous material,` zinciferous ore or ore concentrate, whethernaturally occurring in the oxidized state or obtained by roasting or thelike.V We' have successfully smelted such representative zinciferousmaterials which varied from one extreme to the other in their zinccontent. For example, such oxidic zincifer-` ous materials ascalcinedSterling Hill ore having a 20% zinc content, sintered Eagle oreconcentrate containing about 55% zinc, -sintered Waelz oxide containing'about 70% zinc, and off-grade Zinc oxide pigment containing about 80%zinc,

have been smelted and metallic zinc vapor condensed therefrom withexcellent condensing efficiencies. Inasmuch as our smelting method isapplicable to smelting either the naturally occurring voxidic ore orVroasted sulfide ore, or Such ores which have been subjected to bulkconcentration, it will be observed that the smelting procedure of ourinvention has Yan unlimited applicability as distinguished from the morelimited versatility of other zinc smelting processes. Accordingly, anybeneflciation to which the ore may be subjected prior tosmelting by themethod of our invention need be only for the purpose of removing theeasily separated gangue material by a simple bulk concentration process,Whether by flotation procedure or'otherwise. Y

In smelting pursuant toV ourA invention, the electric arc'should be s0controlled as to obtain a favorable temperature distribution from top tobottom of the furnace. The temperature at the top of the furnace islimited by the refractories used for the furnace structure, but,as aguide it can be stated thatl furnace atmosphere temperatures of1200-1350 C. are satisfactory with temperatures of 12701300 C. beingpresently preferred. .The temperature adjacent'the smelting zone at theinterface betweenV the fiuid slag and floating zinciferous charge shouldnot exceed about 1450" C. as pointed out hereinbefore and preferablyranges between 1350 and 1400 C.

The temperature prevailing through the slag body should be sufficient,however, to maintain both smelting conditions at its surface and molteniron-product conditions below the slag body, taking into account thechilling effector the relatively ccol charge delivered to the surface ofthe slag as well as the endothermic reaction characteristics of thischarge. The position of the 4electric arc required to Aproduce thisresult will depend upon the number of electrodes, furnace size, andother related variables. The electrodes should not be so deeply immersedas to provide simply Vslag-resistance heating for this type of heatinghas been found by us to `be wholly incapable of counteracting thechilling (and slag freezing) eifect of the floating charge. When thefurnace heating iseifected pursuant to the aforementioned specications,Zinc elimination from the charge, as evidenced by the zinc content ofthe slag, will be at least 98 When a zinciferous ore is smelted asdescribed 'hereinbefora the resulting slag consists of lime,

silica and other slag components such as magnesia, manganese oxide,alumina and any unreduced iron oxide and zinc oxide. Of these numerousslag-forming constituents, originating both in the ore and in the ash ofthe carbonaceous reducing material, the lime and silica together usuallycomprise the major portion with the remainder being made up of theaforementioned other slag-forming constituents. In general, the lime andsilica together comprise from about 40% to about 85%, and more commonlyfrom about 60% to about 80%, of the slag. The other slag-formingconstituents, including the maximum of 6% iron oxide (calculated as Fe)which may be permitted to remain in the slag as described hcreinbefore,comprise the remaining to 60%, and more commonly 20 to 40%, of the slag.We have found ythat the slag may be provided with the requisite uidityfor the practice of our sinelting method if a certain `definte-relationship be maintained between the lime-silica ratio and the amountof `said other slag-forming constituents. This relationship isrepresented graphically in Fig. 1 of the drawing.

As indicated in Fig. l, we have found that the aforementioned criticalrelationship holds `true for lime-silica ratios as low as 0.921 and `as4high as 12:1. The minimum lime-silica ratio of0.9:1 represents thelowest ratio at which it is cornmercially feasible to .recover reducedlzinc and iron from the furnace charge. At lime-silica ratios lower than09:1, it becomes so dicult to reduce the zinc and iron that suitableYrecoveries cannotl be obtained at smelting temperatures ranging as highas 1450 C. The upper limit of 1.2:1 for the lime-silica ratio isdictated by .slag viscosity; we have found that slags characterized by alime-silicaratio in excess of 1.2:11 have such viscosity as to renderthem unsatisfactory for smelting operations pursuant 'to our invention.Within these limits, the aforementioned critical relationship betweenthe lime-silica ratio and the amount of the other slag-formingconstituents is represented by the curve AB, the 'lime-'silica ratios onor below the line AB leading to the vproduction of slags havingsuitablefiuidity for practice of our smelting method.

The relationship represented by curve ABin Fig. 1 comprisestheprescription ofthe `maximum lime-silica ratio that can be used withamounts of the other constituents of the slag ranging upwardly fromabout 15% of the slag composition, the lime plus silica constituting thebalance of the slag composition. Although oxidio zinc ores containcalcareous and siliceous materials in a wide range of proportions, wehave observed that it is generally necessary to add lime or silica tothe charge so as to obtain a limesilica ratio in the slag, including thesilica content of the coal ash, coming within the range of 0.9:1 to1.2:1. In order to obtain such a limesilica ratio, extraneous lime orsilica may be added as required, or the requisite lime-silica ratio canbe obtained by the conjoint use Aof two or more znciferous ores ofdifferent calcareous and siliceous contents.

The production of a slag having the requisite fluidity for practice ofour invention can be readily achieved by adjustment of the lime andsilica contents of the sla-g pursuant to a maximum represented by theline AB in Fig. 1. In the case of a lime-base ore, in which thelime-silica ratio exceeds 1.2:1, the silica component of the slag ofsilica or lime which must be added will, of

course, depend -upon the amount of said other slag constituents, asindicated in Fig. 1. lI-Iowever, as clearly shown by the line AB, theaddition of lime or silica, as the case may require, must be made insuch manner as to insure the presence in the resulting slag of at least15% of other slag constituents. That is, in no event should the limeplus silica content of the slag exceed about of the total slagcomposition. The presence of at least 15% other slag constituents canlbe attained by controlling the smelting conditions so as to leave moreiron oxide in the slag, by adding extraneous slag-forming constituentssuch as alumina, magnesia, iron oxide, soda, or the like, or by acombination of such procedures. When using a lime-slag ratio within thelimits prescribed for line AB, we have found that amounts of the otherslag constituents less than about 15% by weight of the slag preclude theattainment of suitable slag fluidity. The upper limit of the amount ofthe other slag constituents is prescribed only by theirnaturallyoccurring proportions in the ore.

Although the establishment of a slag composition characterized by alime-silica ratio not in excess of that represented by the line AB inFig. l will result ina slag fluidity suficiently high to permitsatisfactory smelting in accordance with our invention, we have found`that a more lnarrowly circumscribed relationship Abetween the lime,silica and other slag components leads to the attainment of slags ofmaximum fluidity and maximum utility. rlChese slag compositions whichare characterized by optimum smelting properties are defined by the lineCD in Fig. 1. Theline CD represents the maximum lime-silica Aratiowithin the range of 1.05:1 to 1.15:1,'with respect to the total of theother constituents of the slag, which is compatible with optimum zincand iron recoveries. In obtaining optimum slag uidity pursuant to therelationship represented by line CD, the amount of "other slagconstituents should total at least 17% of the vslag composition.

Slag compositions coming within the vprescription of line AB in Fig. 1,characterized by fluidity `conducive to effective `reduction ofthe zincand iron componentsofthe ore pursuant to our invention, make possiblethe attainment of an efciency of at least 85% in the condensation of theevolved zinc vapor as a result of more complete elimination from thevapor of the aforementioned dust-forming impurities. Slags havingcompositions outside of the prescription of line AB in Fig. 1, on theother hand, are characterized by such inadequate fluidity as to resultin zinc vapor condensation eiciencies (i. e. the proportion of zincvapor introduced into the condenser which is converted to molten metal)materially below 85%. This effect is readily appreciated in thefollowing examples in which smelting conditions pursuant to ourinvention were substantially identical, the difference in each casebein-g essentially a difference in slag composition which, in turn,represented a difference in slag fluidity and consequently in heatdissemination throughout the slag. A slag having a lime-silica ratio of1.2711, and in which the slag constituents v,other than lime and silicaconstituted 20% of the slag composition, resulted in a condensereiiiciency of approximately 80 By increasing the silica content of theslag to obtain a lime-silica ratio of 1.14z1, with a resulting loweringof the amount of other slag constituents to 17% of the slag composition,the condenser eciency was somewhat better than 85%. A further loweringof the limesilica ratio to 104:1 While maintaining the other slagconstituents at 17% raised the condenser eiiiciency to about 90%. Basedupon similar experimental data, we have ascertained that under smeltingconditions pursuant to our invention, as previously set forth, slagcompositions falling within the prescriptions of line AB in Fig. 1 leadto zinc vapor condensation eiciencies in excess of 85% whereas thosecompositions outside of the prescriptions of line AB result in condensereiiiciencies that drop off rapidly to the level of 30% and below. On theother hand, slag compositions coming within the more circumscribedprescriptions of line CD in Fig. 1 make possible zinc vapor condensationefciencies of at least 90%. To the best of our knowledge, the highcondenser emciencies characteristic of smelting operations carried outpursuant to our invention and featuring slag compositions within thelimits Ydiscussed hereinbefore are obtained as the result of theexceptionally uniform temperature which are established throughout suchiluidly mobile slags.

The relatively uniform temperature prevailing throughout the fluid slaglayer is taken advantage of, in accordance with our invention, as ameans of imparting to the fresh charge the necessary smelting heatwithin the specied temperature range. To this end, we have found itadvisable to deliver the charge to the furnace in such manner,advantageously through the furnace roof, as to provide in the vicinityof the electrodes a mass of the charge floating on the slag. Smeltingthus takes place primarily at the interface between the fluid slag andthe floating charge. From time to time an additional charge may beintroduced with advantage adjacent the furnace walls in such manner asto provideY a downwardly and inwardly sloping bank of charge which notonly protects the furnace walls but supplies an additional quantity offresh charge available for absorbing heat from the slag.

The only requirement for the physical form of charge used in practicingour invention is that it be loose and dry. By loose we mean that thecharge should not be introduced in massive form,

conditions say,` as a single large sintered block. The charge shouldA beloose so that it will fall freely on the surface of the molten slag andspread out thereupon to an extent commensurate with the angle of reposeof the charge particles. By specifying that the charge should be dry .Wemean that it should not be added in the molten condition. It is acharacteristic feature of the smelting method of our invention that thecharge be smelted on the surface of the hot uid furnace slag, and thisIcondition can be met only when the 'charge is introduced into thefurnace in the aforementioned loose dry form. The'degree of subdivisionof the ore component of the charge is not critical. In general, weprefer to limit the maximum particle size of the ore in the charge toabout 1/2 inch in diameter. Except for the problem of dusting there isno critical lower limit to the size of any of the charge particles.

We prefer to mix the charge components prior to their introduction intothe furnace either in the form of a simple physical admixture or in theform of nodulized or otherwise agglomerated particles. For example, thezinciferous material (such as Waelz'oxide sinter) coal and lime may bemixed together and charged directly to the furnace, or the mixture maybe moistened with water and further mixed with 2% bentonite as a binder,then briquetted, dried and crushed before charging to the furnace.Alternatively, the zinciferous material alone may be briquetted, usingsulfite liquor as a binder, the briquettes being fired, crushed tosuitable size and then mixed with the coal and lime for direct chargingto the furnace. N o significant differences that could be attributed tothe method of charge preparation were observed in either the smeltingoperation or condenser performance. In each instance, excess nes or thedust fraction may be removed by rough air separation. The charge mayalso be preheated with advantage to temperatures of the order of 400800C. in accordance with conventional electric furnace practice. Anysuitable preheating apparatus may be used for this purpose, the heatbeing supplied by an oil or gas flame or by the heat of combustion ofthe exhaust gases from the zinc condenser.

The amount of coal used in practicing our invention shouldadvantageously somewhat exceed that amount theoretically required forthe zinc and other oxides readily reduced by carbonaceous material. Ingeneral, We have found it satisfactory to use an amount of carbon (coalor coke) calculated to be of that required to reduce the readilyreducible oxides otherthan zinc and about to 125% of the amounttheoretically required to reduce the zinc componentV of the charge.Amounts of carbon in excess of of that theoretically required to reduceall readily reducible oxides in the charge have been found to beineffective in increasing either the total amount of zinc reduced or itsrate of reduction. Amounts of carbon in excess of about 125% of thattheoretically required are undesirable both because of uneconomicalwaste thereof and because of the heat-insulating effect of the excesscarbon floating on the slag. Where coal or coke is used as the reducingmaterial, we have found it advantageous to crush this component ratherthan use it in a relatively coarse form. For exf ample, No.v 3 coaltends to segregate in a zinciferour charge and leads to less effectivereduction than No. 3k coal roll-crushed to through lV mesh (TylerStandard). v

Condensation of the zinc vapor-bearing smelt- 9 i ing gases produced inaccordance with our invention poses no unusual problems. Although astationary baffle-type condenser such as that described in the UnitedStates patent to Bunce No. 1,873,861 can be used for condensing the zincvapor, maximum usufruct of our invention is realized when condensationis effected in a condenser of the type wherein the zinc vapor is broughtinto intimate contact with a relatively large freshly exposed surface ofmolten zinc. The latter type of condenser is represented by that whereinthe zinc vapor-bearing gases are passed through a shower of molten vzincforciblyhurled through a confined condensing zone as described in UnitedStates Patents Nos. 2,457,544 through 2,457,551 and 2,494,551. Thislatter type of zinc condenser is capable of removing and condensing tomolten metal all of the zinc vapor contained in the smelting gasesexcept for that amount of the vapor corresponding to the vapor pressureof g molten zinc at the temperature of the exhaust condenser gases.

The smelting method of our invention may be illustrated by the followingspecific examples. A charge mixture was prepared consisting of about17.8 parts by Weight of anthracite coal (in which silica comprised 65%of the ash) and 1.3 parts by weight of burned lime as an extraneous fluxper 100 parts by weight of a roasted zinc ore concentrate consisting ofdiscrete particles of 1A diameter and finer. The zinc concentrate hadthe following analysis:

Per cent Zn. Pb 0.05 Fe 2.9 Cu 0.02 CaO 2.5 S102 1.6 C 0.80 Mn 1.6

S 0.20 CO2 0.32 Cd 0.002 MgO 0.27 A1203v 0.57

This charge mixture contained sufficient iron oxide and other slagforming gangue constituents to meet the operating requirements of oursmelting method as set forth hereinbefore as can be seen from thefollowing slag analysis:

Per cent 34.8 SiOz 32.3 MgO 2.

- 9.7 CaOzSiOz 1.08 CaO-I-SOz 67.1 Other slag constituents 32.9

The charge materials, thoroughly mixed, were fed to the furnace atintervals of 21/2 minutes successively through a series of six charginghop- 6 trodes, nominally rated at 100 kw. but operated."4

during our smelting operation at only about The two electrodes were sopositioned as to produce arcs adjacent the surface of the slag, the sizeof the arcs at the aforementioned rate of..

power consumption being suflicient to maintain the desired temperatureconditions in the bath and in the furnace chamber above the bath. Thefurnace chamber temperature was maintained in the range of l25G-l300 C.during the operating period. The slag temperature, measured as the slagwas tapped from the furnace, averaged about 13.50 C. rEhe zincVapor-bearing smelting gases were passed through a splash-type condenserof the type referred to hereinbefore and the zinc vapor was condensedtherein to molten metallic Zinc. After correcting for handling losses,for losses of Zinc vapor resulting from furnace leaks, and for theamount of fume passing through the condenser and other similar factors,the condensation efficiency in this operation appeared to besubstantially 90%. The inhibition of volatilization of the refractoryoxides was evident from the fact that the lime-silica ratio in the smallamount of blue powder formed in the condenser was about one-third toone-half of the lime-silica ratio prevailing in the slag.

A second example of the smelting of a zinciferous material in accordancewith our invention comprises the smelting of a furnace charge mixture ofzinc oxide, burned lime and anthracite coal under virtually the samesmelting conditions as those in the previous example. The zinc oxide wasessentially 100% ZnO (79.9 Zn) and comprised a zinc oxide pigmentclassified as offgrade because of its failure to meet certain colorspecifications. Coal additions were made to the extent of 20.1 parts byweight per 100 parts of the zinc oxide to provide the requiredcarbonaceous matter for substantially complete ZnO reduction; the limewas added to the extent of 1.2 parts by Weight per 100 parts of the zincoxide to balance the S102 present inthe coal ash and thereby maintain aC'aOzSiOz ratio in accordance with our invention. The slag was initiallycompounded synthetically to produce in the furnace a molten slag havingthe following composition which, it should be noted, was maintained bythe charge composition:

Per cent Fe 1.4 CaO 42.5 SiOz 40.7 MgCll 7.6 CaO:SiO2 1.04 CaO+SiC2 83.2Other slag constituents 16.8

The iron present in the slag was picked up from the furnace refractoryand from the coal ash, and the magnesia was introduced both as acomponent of the lime addition and by a deliberate ad- .dition to makeup an appropriate amount of other slag constituents.

inoe the zinc oxide pigment in the aforementioned smelting mixturerequired densifying be fore being charged to the smelting furnace, itwas briquetted and the briquettes were crushed to 1/4" particles beforebeing mixed with coal and lime for furnacing. Equally satisfactorycharge preparation was obtained by mixing together and hriquetting thezinc oxide and lime, then crushingl the briquettes, mixing the crushedbriquettes with coal, and rfeeding the resulting mixture to the furnace.The zinc vapor in the resulting snielting gases was condensed with anefficiency cf about 88% in a splash-type condenser. It will he notedthat successful smelting of this zincifcrous charge and effectivecondensation of the resulting Zinc vapor were obtained without produc- 11 ing a metallic iron product as one of the products of the smeltingoperation. In the smelting of such zinciferous material in accordancewith our invention, it must be borne in mind that, with the exception ofthe requirement with respect to the permissible amount of iron oxide inthe slag, each of the other prescribed smelting conditions must beobserved.

It will be seen, accordingly, that our vinvention offers a commerciallyattractive method of smelting oxidic zinciferous materials such asrefuse zinc oxide, blue powder and zinc ores, and the like, in anelectric furnace. The method does not require anything other thanconventional electric furnace equipment for the smelting operation and,in the case of zinciferous ores, not only produces zinc metal but also apig iron product both of which function as collectors for valuable metalby-products which can be readily recovered by conventional means. UnderVnormal operating conditions in a commercial scale furnace there appearsto be every reason to expect a recovery of S-97% of the zinc componentof the ore in the form of condensed molten zinc containing only thoseimpurities which are now removed therefrom by conventionalrectification. In the smelting of zinc ores and the like, substantiallycomplete recovery of the lead, cadmium, copper, silver and gold contentsof the ore can be realized in accordance with our invention, .the

lead, cadmium and some of the silver and gold being recovered from thecondensed metallic zinc and the copper and the balance of the silver andgold being recoverable from the iron product. Only very small amounts ofthese by-product metals are lost to the slag.

We claim:

1. The method of smelting an oxidic zinciferous material with solidcarbonaceous reducing material in an electric arc furnace with theresulting production of a molten slag and metallic zinc vaporsubstantially free from dust-forming impurities which comprises chargingto the furnace the zinciferous material and an amount of thecarbonaceous reducing material exceeding but not more than about 125% ofthe amount of said reducing material stoichiometrically required forreduction of the zinc and other oxides in the charge readily reduced bycarbonaceous material and thereby precluding the presence in the moltenslag of more than 6% by weight of iron oxide (calculated as Fe), furtherincorporating in the charge an amount of an extraneous fluxing materialselected from the class consisting of lime and silica such that uponsmelting of the zinciferous material the resulting molten slag derivedfrom calcareous and siliceous components of the zinciferous material andfrom said extraneous fluxing material will contain not more than 85% byweight of lime (CaO) and silica (Si02) in a ratio between 0.9:1 and1.2:1 and not exceed-y ing, with respect to the total amount of theother constituents of the slag, the ratio represented by the line AB inFig. 1 of the drawing, whereby the fluidity of the slag within thetemperature range of 1250 to 1450u C. is sufficient to permit heating ofthe slag by an electric arc without local overheating of the charge incontact therewith to a temperature in excess of 1150D C., heating saidbody of slag by electric arc heating to a temperature within the rangeof 1250 to 1450 C., and delivering the Ycharge to the furnace in suchmanner as to provide a mass of the charge floating on the slag, smeltingof the charge thus taking place primarily at the inl2 Y terface betweenthe fluid slag and the floating charge by the heat imparted to thecharge by the slag. f Y f 2. The method of smelting an oxidiczinciferous material with solid carbonaceous reducing material'in anelectric arc furnace with the resulting production of a molten slag andmetallic Zinc vapor substantially free from dust-forming impuritieswhich comprises charging to the furnace the zinciferous material and anamount of the carbonaceous reducing material exceeding but not more thanabout 125% Yof the amount of said reducing material stoichiometricallyrequired for reduction of the zinc and other oxides in the chargereadily reduced by carbonaceous material and thereby precluding thepresence in the molten slag of more than 6% by weight of iron oxide(calculated as Fe), further incorporating in the charge an amount of anextraneous iiuxing material selected from the class consisting of limeand silica such that upon smelting of the zinciferous material theresulting molten slag derived from calcareous and siliceous componentsof the zinciferous material and from said extraneous fluxing materialwill contain not more than 83% by weight of lime (CaO) and silica (SiOz)in a ratio between 1.0511 and 1.l5:1 and not exceeding, with respecttothe total amount of the other constituents of the slag, the ratiorepresented by the line CD in Fig. 1 of the drawing, whereby thefluidity of the slag within'the temperature range of 1250 to 1450 C. issucient to permit heating of the slag by an electric arc without localoverheating of the charge in contact therewith to a temperature inexcess of 1450" C., heating said body of slag by electric arc heating toa temperature within the range ofV 1250 to 1450 C., and delivering thecharge to the furnace in such manner as to provide a mass of the chargefloating on the slag, smelting of the charge thus taking place primarilyat the interface between the uid slag and the floating charge by theheat imparted to the charge by the slag. Y, Y

3. The method of smelting an iron-bearing oxidic zinciferous materialwith solid carbonaceous reducing material in an electric arc furnacewith the resulting production of a molten iron product, a molten slagand metallic zinc vapor substantially free from dust-forming impuritieswhich comprises charging to the furnace the zinciferous material and anamount of the carbonaceous reducing material exceeding but not more thanabout 125% of the amount of said reducing material stoichiometricallyrequired for reduction of the zinc and other oxides in the chargereadily reduced by carbonaceous material and thereby precluding thepresence in the molten slag of more than 6% by weight of iron oxide(calculated as Fe), further incorporating in the charge an amount of anextraneous fluxing material selected from the class consisting of limeand silica such that'upon smelting of the zinciferous material theresulting molten slag derived from calcareous and siliceous componentsof the zinciferous material and from said extraneous uxing material willcontain not more than by weight of lime (CaO) and silica (SiOz) in aratio between 0.9:1 and 1.2:1 and not exceeding, with respect to thetotal amount of the other constituents of the slag, the ratiorepresented by the line AB in Fig. 1 of the drawing, whereby the uidityof the slag within the temperature range of 1250 to l450 C. Yis sufcientto permit heating of the slag by an electric arc Without localoverheating of the charge in contact therewith to a temperature inexcess of 1450 C., heating said body of slag by electric arc heating toa temperature within the range of 1250 to 1450 C., and delivering thecharge to the furnace in such manner as to provide a mass of the chargefloating on the slag, smelting of the charge thus taking place primarilyat the interface between the fluid slag and the floating charge by theheat imparted to the charge by the slag.

4. The method of smelting an iron-bearing oxidic Zinciferous materialwith solid carbonaceous reducing material in an electric arm furnacewith the resulting production of a molten iron product, a molten slagand metallic zinc vapor substantially free from dust-forming impuritieswhich comprises charging to the furnace the zinciferous material and anamount of the carbonaceous reducing material exceeding but not more thanabout 125% of the amount of said reducing material stoichiometricallyrequired for reduction of the zinc and other oxides in the chargereadily reduced by carbonaceous material and thereby precluding `thepresence in the molten slag of more than 6% by weight of iron oxide(calculated as Fe), further incorporating in the charge an amount of anextraneous fluxing material selected from the class consisting of limeand silica such that upon smelting of the zinciferous material theresulting molten slag derived from calcareous and siliceous componentsof the zinciferous material and from said extraneous fluxing materialwill contain not more than 83% by weight of lime (CaO) and silica (S102)in a ratio between 1.05:1 and 1.15:1 and not exceeding, with respect tothe total amount of the other constituents of the slag, the ratiorepresented by the line CD in Fig. 1 of the drawing, whereby thefluidity of the slag within the temperature range of 1250 to 1450c C. issufcient to permit heating of the slag by an electric arc Without localoverheating of the charge in contact therewith to a temperature inexcess of 1450 C., heating said body of slag by electric arc heating toa temperature within the range of 1250 to 1450 C., and delivering thecharge to the furnace in such manner as to provide a mass of the chargefloating on the slag, smelting of the charge thus taking place primarilyat the interface between the fluid slag and the floating charge by theheat imparted to the charge by the slag.

5. The method of smelting an iron-bearing oxidic zinciferous materialwith solid carbonaceous reducing material in an electric arm furnacewith the resulting production of a molten iron product, a molten slagand metallic zinc vapor substantially free from dust-forming impuritieswhich comprises charging to the furnace the zinciferous material and anamount of the carbonaceous reducing material exceeding but not more thanabout of the amount of said reducing material stoichiometricallyrequired for reduction of the zinc and other oxides in the chargereadily reduced by carbonaceous material and thereby precluding thepresence in the molten slag of more than 6% by weight of iron oxide(calculated as Fe), further incorporating in the charge an amount of anextraneous uxing material selected from the class consisting of lime andsilica such that upon smelting of the zinciferous material the resultingmolten slag derived from calcareous and siliceous components of thezincifero/us material and from said extraneous fluxing material willcontain not more than 83% by weight of lime (CaO) and silica (SiOz) in aratio between 1.05:1 and 1.15:1 and not exceeding, with respect to thetotal amount of the other constituents of the slag, the ratiorepresented by the line CD in Fig. 1 of the drawing, whereby thefluidity of the slag within the temperature range of 1250o to 1450 C. issuiclent to permit heating of the slag by an electric arc without localoverheating of the charge in contact therewith to a temperature inexcess of 1450 C., heating said body of slag by electric arc heating toa temperature within the range of 1250 to 1450* C., delivering thecharge to the furnace in such manner as to provide a mass of the chargeoating on the slag, smelting of the charge thus taking place primarilyat the interface between the uid slag and the oating charge by the heatimparted to the charge by the slag, and condensing the resulting zincvaporbearing gases in a condensing zone by subjecting them to intimatecontact with a relatively large freshly exposed surface of molten zincmetal.

ROBERT K. WARING. LUTHER D. FETTEROLF. THOMAS L. HURST.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 834,644 Snyder Oct. 30, 1906859,132 SnyderV July 2, 1907 933,133 Snyder Sept. 7, 1909 1,738,910Lepsoe Dec. 10, 1929 2,457,544 Handwerk et al. Dec. 28, 1948 2,509,326Weaton et al. May 30, 1950 OTHER REFERENCES The Art of Electric ZincSmelting by Johnson, published 1913 by Continuous Zinc Furnace Co.,Hartford. Conn.

1. THE METHOD OF SMELTING AN OXIDIC ZINCIFEROUS MATERIAL WITH SOLIDCARBONACEOUS REDUCING MATERIAL IN AN ELECTRIC ARC FURNACE WITH THERESULTING PRODUCTION OF A MOLTEN SLAG AND METALLIC ZINC VAPORSUBSTANTIALLY FREE FROM DUST-FORMING IMPURITIES WHICH COMPRISES CHARGINGTO THE FURNACE THE ZINCIFEROUS MATERIAL AND AN AMOUNT OF THECARBONACEOUS REDUCING MATERIAL EXCEEDING BUT NOT MORE THAN ABOUT 125% OFTHE AMOUNT OF SAID REDUCTION MATERIAL STOICHIOMETRICALLY REQUIRED FORREDUCTION OF THE ZINC AND OTHER OXIDES IN THE CHARGE READILY REDUCED BYCARBONACEOUS MATERIAL AND THEREBY PRECLUDING THE PRESENCE IN THE MOLTENSLAG OF MORE THAN 6% BY WEIGHT OF IRON OXIDE (CALCULATED AS FE), FURTHERINCORPORATING IN THE CHARGE AN AMOUNT OF AN EXTRANEOUS FLUXING MATERIALSELECTED FROM THE CLASS CONSISTING OF LIME AND SILICA SUCH THAT UPONSMELTING OF THE ZINCIFEROUS MATERIAL THE RESULTING MOLTEN SLAG DERIVEDFROM CALCAREOUS AND SILICEOUS COMPONENTS OF THE ZINCIFEROUS MATERIAL ANDFROM SAID EXTRANEOUS FLUXING MATERIAL WILL CONTAIN NOT MORE THAN 85% BYWIEGHT OF LIME (CAO) AND SILICA (SIO2) IN A RATIO BETWEEN 0.9:1 AND1.2:1 AND NOT EXCEEDING, WITH RESPECT TO THE TOTAL AMOUNT OF THE OTHERCONSTITUENTS OF THE SLAG THE RATIO REPRESENTED BY THE LINE AB IN FIG. 1OF THE DRAWING, WHEREBY THE FLUIDITY OF THE SLAG WITHIN THE TEMPERATURERANGE OF 1250* TO 1450* C. IS SUFFICIENT TO PERMIT HEATING OF THE SLAGBY AN ELECTRIC ARC WITHOUT LOCAL OVERHEATING OF THE CHARGE IN CONTACTTHEREWITH TO A TEMPERATURE IN EXCESS OF 1450* C., HEATING SAID BODY OFSLAG BY ELECTRIC ARC HEATING TO A TEMPERATURE WITHIN THE RANGE OF 1250*TO 1450* C., AND DELIVERING THE CHARGE TO THE FURNACE IN SUCH MANNER ASTO PROVIDE A MASS OF THE CHARGE FLOATING ON THE SLAG, SMELTING OF THECHARGE THUS TAKING PLACE PRIMARILY AT THE INTERFACE BETWEEN THE FLUIDSLAG AND THE FLOATING CHARGE BY THE HEAT IMPARTED TO THE CHARGE BY THESLAG